In this longitudinal field study, we examine reciprocal relationships between within-person changes in work engagement and cognitive appraisals of change (threat and challenge) across an organizational merger. Examination of these cyclical relationships provides a more accurate understanding of the complexity of employees' experience of change and a new test of spiraling work engagement and cognitive appraisals. Latent change score modeling is used to analyze 3 waves of longitudinal survey data (N = 623). Our findings showed that engagement mitigated threat appraisals and enhanced challenge appraisals through pre- and postmerger phases. A reciprocal relationship between threat appraisal and engagement was also observed, such that threat fueled decreases in engagement throughout the merger. Challenge appraisal was associated with enhanced work engagement during the first merger phase. This examination advocates managers of change to foster employees' work engagement already prior to change endeavors, along with mitigating threat appraisals throughout organizational change events. Fostering positive challenge appraisals appears to be particularly important for employees' work engagement during times of major changes. Findings suggest that upward spiral of work engagement, as postulated on the basis of the broaden-and-build theory, may be more likely to occur through engagement mitigating negative cognitions (threat) than promoting positive cognitions (challenge). (PsycInfo Database Record (c) 2020 APA, all rights reserved).